The present invention is generally directed to cloth-like nonwoven webs. More particularly, the present invention is directed to a process for increasing the softness and decreasing the stiffness of nonwoven webs made from thermoplastic polymers and to a composition which produces softer webs with low luster.
Many woven and nonwoven webs and fabrics are formed from thermoplastic polymers, such as polypropylene and polyethylene. For instance, spunbond webs, which are used to make diapers, disposable garments, personal care articles, and the like, are made by spinning a polymeric resin into fibers, such as filaments, and then thermally bonding the fibers together. More particularly, the polymeric resin is typically first heated to at least its softening temperature and then extruded through a spinnerette to form fibers, which can then be subsequently fed through a fiber draw unit. From the fiber draw unit, the fibers are spread onto a foraminous surface where they are formed into a web of material.
Besides spunbond webs, other fabrics made from polymers include meltblown fabrics. Meltblown fabrics are made by extruding a molten polymeric material through a die to form fibers. As the fibers exit the die, a high pressure fluid, such as heated air or steam, attenuates and breaks the fibers into discontinuous fibers of small diameter. The fibers are randomly deposited onto a foraminous surface to form a web.
Spunbond and meltblown fabrics have proven to be very useful for many diverse applications. In particular, the webs are often used to construct liquid absorbent products, such as diapers, feminine hygiene products, and wiper products. The nonwoven webs are also useful in producing disposable garments, various hospital products, such as pads, curtains, and shoe covers and recreational fabrics, such as tent covers. Although well suited for these applications, recently, attention has focused on making the nonwoven webs more cloth-like in order to avoid the plastic-like feel and look of such fabrics. Cloth, as opposed to plastic fabrics, has a more pleasing appearance and feel.
In the past, various attempts have been made to produce more cloth-like fibers from plastic materials in order to produce fibrous webs. For instance, in U.S. Pat. No. 4,254,182 to Yamaguchi, et al., polyester synthetic fibers are disclosed having an irregular uneven random surface formed by microfine recesses and projections to provide more natural feeling fibers. The microfine recesses and projections are produced by incorporating into the fibers silica in a size ranging from 10 to 150 microns and in an amount so as to produce surface projections. It is taught that the surface projections effectively increase the surface area of the fibers and contribute to greater frictional forces, which reduce the slick, waxy feel that is typically associated with plastic resins.
The prior art, however, merely teaches increasing the frictional characteristics of the polymeric fibers in order to remove the wax-like feel of plastics. A need remains for a method that will alter the physical properties of the fibers so that webs made from the fibers will feel more cloth-like and have other cloth-like characteristics. In particular, a need exists for more cloth-like fibrous webs and laminates thereof made from thermoplastic fibers that are less stiff and softer than conventionally made webs.
As used herein the term xe2x80x9cnonwoven fabric or webxe2x80x9d means a web having a structure of individual fibers or threads which are interlaid, but not in an identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes, such as for example, melblowing processes, spunbonding processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters useful are usually expressed in micros. (Note that to convert from osy to gsm, multiply osy by 33.91).
As used herein the term xe2x80x9cspunbond fibersxe2x80x9d refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel, et al., and U.S. Pat. No. 3,692,618 to Dorschner, et al., U.S. Pat. No. 3,802,817 to Matsuki, et al. U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinnery, U.S. Pat. No. 3,502,763 to Hartman, U.S. Pat. No. 3,502,538 to Levy, and U.S. Pat. No. 3,542,615 to Dobo, et al. Spunbond fibers are generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and have diameters larger than 7 microns, more particularly, between about 10 and 20 microns.
As used herein the term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity gas (e.g. air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in diameter, and are generally tacky and self adherent when deposited onto a collecting surface.
As used herein the term xe2x80x9cpolymerxe2x80x9d generally includes but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof.
As used herein, the term xe2x80x9cmachine directionxe2x80x9d or MD means the length of a fabric in the direction in which it is produced. The term xe2x80x9ccross machine directionxe2x80x9d or CD means the width of fabric, i.e. a direction generally perpendicular to the MD.
As used herein the term xe2x80x9chomopolymerxe2x80x9d fiber refers to the fiber or part of a fiber formed from one extruder using only one polymer. This is not meant to exclude fibers formed from one polymer to which small amounts of additives have been added for coloration, anti-static properties, lubrication, hydrophilicity, etc. These additives, e.g. titanium dioxide for coloration, are generally present in an amount less than 5 weight percent and more typically about 2 weight percent. The term xe2x80x9chomopolymerxe2x80x9d is also not meant to exclude a fiber formed from two or more extruders wherein both of the extruders contain the same polymer.
As used herein the term xe2x80x9cbicomponent fibersxe2x80x9d refers to fibers which have been formed from at least two polymers extruded from separate extruders but spun together to form one fiber. Bicomponent fibers are also sometimes referred to as multicomponent fibers. The polymers are usually different from each other though bicomponent fibers may be homopolymer fibers. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the bicomponent fibers and extended along the length of the bicomponent fibers. The configuration of such a bicomponent fiber may be, for example, a sheath/core arrangement wherein one polymer is surrounded by another or may be a side by side arrangement or an xe2x80x9cislands-in-the-seaxe2x80x9d arrangement. Bicomponent fibers are taught in U.S. Pat. No. 5,108,820 to Kaneko, et al., U.S. Pat. No. 5,336,552 to Strack, et al., and European Patent No. 0586924. For two component fibers, the polymers may be present in ratios of 75/25, 50/50, 25/75 or any other desired ratios.
As used herein the term xe2x80x9cbiconstituent fibersxe2x80x9d refers to fibers which have been formed from at least two polymers extruded from the same extruder as a blend. The term xe2x80x9cblendxe2x80x9d is defined below. Biconstituent fibers do not have the various polymer components arranged in relatively constantly positioned distinct zones across the cross-sectional area of the fiber and the various polymers are usually not continuous along the entire length of the fiber, instead usually forming fibrils or protofibrils which start and end at random. Biconstituent fibers are sometimes also referred to as multiconstituent fibers. Fibers of this general type are discussed in, for example, U.S. Pat. No. 5,108,827 to Gessner. Bicomponent and biconstituent fibers are also discussed in the textbook Polymer Blends and Composites by John A. Manson and Leslie H. Sperling, copyright 1976 by Plenum Press, a division of Plenum Publishing Corporation of New York, IBSN 0-306-30831-2, at pages 273 through 277.
As used herein the term xe2x80x9cblendxe2x80x9d means a mixture of two or more polymers while the term xe2x80x9calloyxe2x80x9d means a sub-class of blends wherein the components are immiscible but have been compaticilized. xe2x80x9cMiscibilityxe2x80x9d and xe2x80x9cimmiscibilityxe2x80x9d are defined as blends having negative and positive values, respectively, for the free energy of mixing. Further, xe2x80x9ccompatibilizationxe2x80x9d is defined as the process of modifying the interfacial properties of an immiscible polymer blend in order to make an alloy.
The present invention recognizes and addresses the foregoing drawbacks and deficiencies of prior art constructions and methods.
Accordingly, it is an object of the present invention to provide an improved composition for producing more cloth-like fibrous webs from thermoplastic polymers.
It is another object of the present invention to provide more cloth-like fibers, including filaments, made from thermoplastic polymers.
It is another object of the present invention to provide more cloth-like nonwoven webs and laminates thereof made from thermoplastic polymers that have stiffness and softness characteristics that are comparable to fabrics made from natural fibers.
Still another object of the present invention is to provide more cloth-like fibers, webs and laminates made from thermoplastic polymers by incorporating into the polymers a mixture of fillers.
Another object of the present invention is to provide more cloth-like fibers, webs and laminates made from a thermoplastic polymer by incorporating into the polymer a mixture of mineral fillers, such as kaolin clay or calcium carbonate, and titanium dioxide.
These and other objects of the invention are achieved by providing a process for producing more cloth-like nonwoven webs from polymeric fibers with improved visual aesthetics. The cloth-like properties are produced by incorporating a mixture of fillers into a thermoplastic polymeric material. The mixture of fillers includes titanium dioxide and a mineral filler. The mineral filler is preferably calcium carbonate or kaolin clay. Other mineral fillers that may be used in the process include talc, gypsum, diatomaceous earth, other natural or synthetic clays, and mixtures thereof. Particular clays that may be used in the present invention besides kaolin, include attapulgite clay, bentonite clay, or montomorillonite clay.
Once the fillers are incorporated into the thermoplastic polymeric material, the polymer is formed into fibers. The fibers are then subsequently used to create a nonwoven web. The mixture of fillers incorporated into the polymeric material is added in an amount sufficient to decrease the stiffness and increase the softness of the web in comparison to nonwoven webs made from the thermoplastic polymeric material not containing any fillers.
In most applications, according to the present invention, the fibers are formed by extruding the thermoplastic polymeric material. For instance, the nonwoven web can be made from meltblown fibers or spunbond fibers. The thermoplastic polymeric material used to make the fibers can be, for instance, a polyolefin, a polyamide, such as nylon, a polyester, a mixture of the above polymers, and copolymers of the above polymers such as copolymers comprising propylene units. In one embodiment, the thermoplastic polymer is polypropylene or a copolymer containing polypropylene.
The amount of fillers added to the thermoplastic polymeric material will generally depend upon the particular application. For most applications, the mineral filler should be added to the polymeric material in an amount up to about 10% by weight, while the titanium dioxide can be added to the polymeric material in an amount up to about 4% by weight. More particularly, for most applications, the mineral filler will be added to the polymeric material in an amount from about 2.5% by weight to about 5% by weight, while the titanium dioxide will be added in an amount from about 1% by weight to about 2% by weight. In general, the fillers should be added to the polymer in an amount insufficient for the fillers to substantially protrude from the surface of the fibers. For instance, the surface of the fibers should not become rough due to the presence of the fillers.
In order to incorporate the fillers into the thermoplastic polymer, the fillers can be added to the polymer in combination with a vehicle, such as a low molecular weight wax. For example, in one embodiment, the vehicle can be a wax that is blended with the fillers prior to being added to the polymeric material. The wax can be, for instance, a low density, low molecular weight, polyethylene or polypropylene. The wax can be mixed with the fillers in an amount of about 50% by weight.
According to the present invention, it has been discovered that when a mineral filler in combination with titanium dioxide is added to a thermoplastic polymer during the formation of nonwoven webs, the webs have improved cloth-like properties, improved luster, and less gloss. For instance, it has been discovered that the nonwoven webs are softer and less stiff. The fillers also only minimally affect the strength or abrasion resistance of the nonwoven web or the fibers used to make the web. It has been further discovered that the fillers also improve the thermal aging stability of the web, which refers to the ability of the web to withstand high temperatures for a prolonged period of time without degrading.
These and other objects of the present invention are also achieved by providing fibers and webs made from the fibers. The fibers produced according to the present invention are designed to produce cloth-like webs useful for many diverse applications. The fibers are made from a thermoplastic polymer containing a mixture of fillers. The fillers include titanium dioxide and at least one mineral filler. The fillers are encapsulated within the thermoplastic polymer and are added in an amount insufficient for the fillers to protrude from the surface of the fibers.
The fibers produced can be discontinuous or continuous fibers and can be made according to a meltblown process or a spunbond process.
Other objects, features, and aspects of the present invention will be discussed in greater detail below.